The reporters for the Unity Weekly News were convicted by a court in central Magway region under the former junta-run country's official secrets act, amid concerns that Myanmar is backsliding on press freedoms.
"All five journalists were sentenced to 10 years imprisonment," said lawyer Wah Win Maung, who is acting on behalf of four of the men.
"This verdict is legally wrong. We will appeal," he told AFP.
The jailed men, who include the chief executive of the Unity Weekly News and are aged between 22 and 52, were arrested in February just days after the article was published.
Reporters Without Borders described the verdict as "very worrying for press freedom" in Myanmar.
"It's clearly a step back," said Benjamin Ismail, head of the watchdog's Asia-Pacific desk.
The article alleged that the country's military was operating a chemical weapons factory in the town of Pauk in Magway, under the instructions of former strongman junta chief Than Shwe.
Myanmar's military ruled the country with an iron fist for nearly five decades, banning dissent and imprisoning critics and journalists.
- Media crackdown -
The new quasi-civilian regime that came to power in 2011 has won praise with reforms including freeing political prisoners and lifting draconian pre-publication censorship.
But rights groups have highlighted mounting concerns over press freedoms, after several cases of criminal prosecutions against journalists.
David Mathieson, a researcher with New York-based Human Rights Watch, said the Myanmar media had come under increasing pressure in recent months, with several high profile arrests of reporters and planned new media laws that have raised concerns.
"The government is cracking down on the media doing its job," he said.
He noted that the Unity Weekly case was complicated by accusations of trespassing and revealing state secrets, which are illegal in many countries.
"But that does not excuse the fact that the sentences were outrageously harsh," Mathieson said, adding that the reporters could have had a public interest defence based on Myanmar's commitment not to use chemical weapons.
Unity Weekly quoted testimony from local people and workers and included pictures of the alleged military facility in its article.
In January last year Myanmar denied accusations it had used chemical weapons against ethnic minority rebels in the northern state of Kachin, with government spokesman Ye Htut saying the military "never" deployed them.
The US Treasury in December levelled sanctions against a Myanmar military official and three businesses in the country for trading arms with North Korea.
At the same time the Treasury said the sanctions, which forbid any US person or entity from doing business with those blacklisted, did "not generally target" the government of Myanmar.
In April a reporter for the Democratic Voice of Burma, Zaw Pe, was jailed for one year -- together with the father of a student -- for "disturbing a civil servant" and trespassing after he tried to interview an education official.
The men have since been released after their jail terms were reduced on appeal.